Electromagnetic transducer head assembly



Jan. 16, 1951 ZENNER 2,538,405

ELECTROMAGNETIC TRANSDUCER HEAD ASSEMBLY Filed April 27, 1948 2 Sheets-Sheet 1 Patented Jan. 16, 1951 ELECTROMAGNETIC TRANSDUCER .HEAD ASSEMBLY Raymond. E. Zenncr, Brookfield, Ill., assignor to Armour Research Foundation. of Illinois Institute of "Technology,

tion of Illinois Chicago, 111., a corpora- Application April27, 1948, Serial No.-.23,507r

7 Claims. 1,

My invention relates. to. an electromagnetic transducer head assembly and more-particularly to an electromagnetic transducer. head. assembly especially suitable. for. converting variations in the degree of magnetization along. the. lengthof al'engthy magnetizedrecord medium tov a-timevarying. electromotive force...

In one formof recordi'ngan intelligence. on..a lengthy magnetizable. recordmedium, the. medium is passedacross an. electromagnetic transducer head to impartvariations inthe degree of magnetization along. the length thereof in accord withthe time variations ofithev intelligence. This electromagnetic transducer head'may include a core member havingconfronting pole pieces defining a nonmagnetic .gap'and across which the lengthy magnetizable record medium is caused to travel; A coil encircles this core member and currentis caused to flow, therethrough in accord with the time variationsof the intelligence, thus causing-a time-varying magnetic field across the air gap. As the medium is drawn across the confronting pole pieces; successive incremental portions" thereof are subjected to the flux set up at the air gap at the time of traveling thereover, thereby causing the medium to be magnetized along its length in accord w'ith the time variations of'the' intelligence;

During the reproducing operation, the magnetized record medium' is drawn across the same or a similar electromagnetic transducer headas sembly in like fashion. In this case, however, as each incremental portion of the medium: passes across the non-magnetic'gap, magnetic'fiux is produced in: the corein accord with the degree of magnetization of that incremental portiom Thus; the flux int-he core experiences time variat-ions in accord with thepassage or the medium thereoverand'the' time variations of the intelligence recorded=on the medium; This time-varyingzflux produces an inducedfvoltag'e in' the" coil encircling the core "member, which ivoltage'maybe amplified and converted by a loudspeaker or similardeviceto the originalintelligence such as; for example, sound waves.

ltyisawell known that: for perfect fidelity of operation the various frequency components of the recorded intelligenceshould be reproduced with like intensity. Toachieve this end" in a practical manner, it-is desirable in magnetic re cording. and: reproducing mechanisms to prevent variations" inv the intensityof" the reproduced in telligenceoversmall changes-in frequency; This is. a consequence; ofthe fact that whilev uniform gradual changes ovena wide: frequency range-nan be overcome .by suitable equalizer networks, sud-.- den and abrupt changesare difficult or: impossible to compensate.

It is further necessary int electromagnetic transducer headassemblies toprovide a mechanism that is relativelyinsensitive -to stray alternating magnetic fieldssuch as those associated with the operation of power transformersalterhating current motors, andthe like. Such stray magnetic fields are exceedinglydiffioultto avoid in magnetic recordersand reproducers and most effective operation demands-that their effects on the reproduced intelligence beminimized.

Still. another desirable feature of an electromagnetic transducer headis that the, reluctance of the iron be relatively small as comparedwith the. non-magneticgap while at thesame time the non-magnetic gap is ,of. small size .for good. resolution. Moreoven. auxiliary non-magnetic gaps shouldbe avoided .asithesereduce the outputvoltage, of the. head, thereby. increasing. the. noise level ofv the, reproduced intelligence and requiring an increased degree of amplification.

It is therefore a general obiectoithe present invention to provide an. improved electromagnetic transducer head assembly.

Furthenit. is an object. of thepresent inven: tion to provide an improved. electromagnetic transducer head assembly wherein the output voltage varies in a. uniform manner with frequency.

Yetanother object of. the present inventionis to. provide an improved electromagnetic transducer head'assembly that is inherently insensitive to. stray magnetic fields.

An additional object of the present invention is to provide an improved electromagnetic'transducer head assembly having the foregoing ad+ vantages and which; in addition, requires only a single non-magnetic gap and in which thereluctance of theiron is relatively small as compared with the non-magnetic gap.

My invention further resides in features of construction, combination and arrangement whereby a new and improved electromagnetic transducer head assembly that is simple in construction and reliable in operation isprovided to the end that a unit of maximum utility is achieved.

The novel features which I'believe'to be characteristic of'my invention are set forth with particularity in the appended-claims. My invention itself, however,- both as to its organization and method" of operation; together with further-ob jects andaadvantages'thereof; may b'est b'e understood by reference to the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a diagrammatic top plan view of a complete magnetic recorder incorporating an electromagnetic transducer head assembly of the type contemplated by the present invention;

Figure 2 is a diagrammatic view illustrating how the electromagnetic transducer head assembly of the present invention is insensitive to magnetic variations along the lengthy magnetizable medium as they approach the head;

Figure 3 is a diagrammatic view illustrating how the electromagnetic transducer head assembly of the present invention is sensitive to magnetic variations along the record medium extending over the non-magnetic gap portions thereof;

Figures 4 and 5 are side and end views respectively of a complete electromagnetic transducer head assembly constructed in accordance with the principles of the present invention;

Figure 6 is an isometric view of the head shown in Figures 4 and 5;

Figures 7 and 8 are charts illustrating actual performance achieved with an electromagnetic transducer head like that shown in Figures i, 5 and 6; and

Figure 9 is a view of an alternative embodiment of the present invention.

In Figure 1 there is shown a top plan View of a complete magnetic recorder incorporating an electromagnetic transducer head constructed in accordance with the principles of the present invention. As shown in this view, the assembly includes a top panel l upon which a pair of spaced disks [2 and M are rotatably mounted in spaced position relative to each other with their axes parallel. Mounted on the disks l2 and H! are a pair of spools l3 and I upon which is wound lengthy magnetizable medium such as a wire [6. The wire [0 extends between spools l3 and I5 and is transferable therebetween by rotations thereof.

An electromagnetic transducer head assembly is is disposed between the spools l3 and iii to engage the medium H5 in the region that it travels therebetween. This head includes a magnetic core defining the non-magnetic gap 22 across which medium l6 travels and a pair of magnetic circuits including this gap as a portion thereof. One of these circuits is formed by the U-shaped portion of the core 20 and 'is linked by the bucking winding 24. The other circuit is formed by the O-shaped portion 2! of the core 20 and is linked by the signal winding 26. Windings 24 and 26 are connected in series relationship across the magnetic reproducer unit 21.

During operation of the mechanism of Figure 1, the disk M- is rotated by suitable mechanical drive elements (not shown) to rotate spool I5 and cause travel of medium l6 in the direction of arrow 28. The disk !2 is retarded in its rotation by a suitable brake, thus retarding spool 13 to maintain the medium l6 taut in the region between the spools and across the head l8. As the medium it travels across the head Hi, the quantities of magnetic flux linked by the coils 2B and 2A! are changed relative to each other, thereby causing a net induced voltage across the series connected coils which is applied to the reproducer 2]. The reproducer 21 includes an amplifier to increase the power level of the inducedvoltage across the coils 24 and 26, together with 4 a suitable converting device such as, for example, a loud speaker. Thus, as the induced voltage in coils 24 and 26 varies with time in accordance with variations in the degree of magnetization of medium It along its length, the resultant voltage is amplified and converted to the original intelligence, namely, sound.

In a practical head constructed as shown in Figure l, the signal coil 26 might have 40 turns and the bucking coil 24 might have 26 turns. The core 20 might comprise a single lamination about 0.015 inch thick and the various portions of the core might be about 0.031 inch wide, giving about 0.0005 square inch cross-sectional area. The O-shaped core portion 26 might have an outer radius of about 0.125 inch and the U-shaped portion 25 might define a rectangleabout 0.5 inch by 0.75 inch with the edge adjacent nonmagnetic gap 22 tapered off along a radius of about 1 /2 inches. The coil 24 is wound in bucking relationship with coil 20 so that increasing flux in like direction through these coils induces opposing voltages.

In Figure 2 there is shown a portion of a lengthy bar magnet 34 defining a magnet pole positioned adjacent the corner of the head [8. This magnet is representative of the situation existing when a magnetic variation in the medium l6 approaches the head l8 from one side. As indicated by the dashed lines and the arrows 30 and 32, this magnet pole tends to cause flux passage through the bottom portion 20a and the top portion 20b of the core 20 in like direction, so that as the pole 34 approaches the head I8 the flux increase in like direction linking coils 24 and 20 takes place. Consequently, opposing voltages are induced in these coils and the voltage applied to the reproducer 21 is much less than if either coil 24 or coil 26 alone is used.

It will be observed that with the representative magnet 34 in the position shown, the length of the flux path followed by flux threading coil 24 is comparable in length to the flux path followed by flux threading coil 26. While it is difficult to predict exactly all the paths followed by the various flux elements and the quantity of flux in each path, it will be evident that a pro- DOltiOIllllo; of the numbers of turns in windings 24 and 25 will give no net voltage at reproducer 21 for one position of the representative magnet 34.

In Figure 3 there is shown the head I8 with a representative bar magnet 36 positioned directly across the air gap 22. This magnet represents the effect of the portion of the medium [6 (Figure 1) located directly over the air gap 22. As will be evident from Figure 3, flux from the representative magnet follows two paths, one path linking the coil 24 as indicated by the arrow 38 and the other path linking the coil 26 as indicated by the arrow 40. Thus, the magnet acts to cause magnetic flux linking coils 24 and 26 in the same direction as does the magnet 34 (Figure 2).

It will be noted, however, that the length of the flux path followed by the flux encircling the coil 24, as indicated by the arrow 38 (Figure 3), is very much greater in length than the path followed by the flux indicated by the arrow 40 and linking the coil 26. Since these two flux paths are effectively in parallel insofar as the magnet 36 is concerned, it will be evident that the quantity of the fillX' following the pathof 7 arrow 40 is greatly in excess of the quantity of flux: following: the path of arrow; 38;; CURSE." quently, if the relative numbers of turns; on coils 24 and 26 are proportioned: to givexa; small or: no. resultant induced: voltage in. the case of the. .representative.magnet of Figure 2; there will be a; substantial induced. voltage in. the case of themagnet; 36. of. Figure 3.

From: the foregoingitwill be evident that; as a: magnet pole in; the medium [6 traverses the head- !85. the value of the flux encircled by coil 2.4."is varied relative to the; value of theflux en: circled by -the coilj25. Hence, the induced-voltages; in these coils are determined by the time rate of change of theseseparate fluxes, andthe net=.voltage-appearing across-the reproducer 2'! is, correspondingly varied;- When. the magnet pole approaches; and recedes: from: the; head; these inducedvoltagesare nearly: in balance and little or'no: net/voltage is, produced: However, when: the magnet pole is; over the non-magnetic gap- 22; these: voltages are out of balance; and a substantial. net. induced voltage is produced.

Stray alternating magnetic .fields such as those associated; with transformers, alternating current motors; and the like, cause magnetic fields of; force of substantially constant direction over the: effective; dimensions of." the electromagnetic transducer head l8. Consequently, these fields tendhto produce: magnetic-flux linkingthe windings: 24: and 26' in like direction and thus actmnch as the flux'from the barmagnet 34 (Fig- 111'62) inproducing opposed electromotive forces giving no: net voltage for application to reproducerxZrli;

Figures; and 5 are side elevational and end elevationalviews respectively of a complete electromagnetic transducer head assembly constructed in accordance with the principles of the present invention and intended for use in connection with a length magnetizable medium such as wire. Thisassembly includes a core portion 21] having U-shaped lower section with its bottom leg encircled by the coil 24 and an O-shaped section encircled by the coil. These two sections combine todefine a common air gap 22. The core-portion 20 is positioned between two spaced non-magnetic members 40 and 42 which areurged against each other by the-screws 44. At" their upper portions these non-magnetic members define a V-shaped notch 46 to receive the lengthymagnetizable record medium. The core20 is provided with aVslot-48 in aligned relationship with the V-notch 46. to receive the medium IS; The. V-notch 46 acts to retain the medium l6 when a knot or other obstruction in the same causes the latter to ride out of the groove 48;

The blocks 40 and 42 are provided with windows; 5B and" 52. to receive the windings 24' and Efig respectively.

Figure 6.71s an isometric view of the complete electromagnetic transducer head. shown. in Figures land 5.

Itwillbe observed from the views ofthe figures that the core member 20 defines, tWO magnetic circuits haying the common non-magnetic gap 22 Moreoventhe confiuences or points of connection. of. these two circuits. are. near. the. commen @1122; One of these circuits, namely, the U.-shaped ircuit including theportions. 20a and 2.013., (Figure. 2).. and.v the. side,v portions, is, of rela:

tively greatlen th. and. in. addition, itdefines portions. adacent. the air: gap, 22, to. receive. the. medium. IS. The. other or these circuits, is of or. falls in this line.

8 relatively" short. length and is relatively remote from the medium [6, throughoutits length;

Figure 7, curve A, shows an output characteristic of anelectromagnetic'transducer head such as that shown in'Figures 4 to 6. This characteristic was obtained by measuring-theelectromotive force applied to the reproducer2'l as the medium I6 was drawnat uniform velocity across the head. The medium I6 actually used for this purpose had variations in magnetization imparted along its length corresponding to a range of frequencies; theconstant current imparted in the case-of each frequency being held constant.

It willbe observed that the output voltage of the head l8 (cur.ve=A), asmeasured on the decibel. scale in Figure '7, forms a substantially straight line in the region from 40to 3000 cycles, and. that there are no substantial sudden rises This line corresponds to the theoreticalv response curve corresponding to low frequencies, and in a practical reproducer 21. thevcltage may be made effectively constant by utilizing a suitable integrating orequalizing network. Above 3000 cycles it will be observed that the output voltage drops ofif in a substan tially even and uniform fashion. In this-region it is likewise possible to provide a suitable equalizingnetwork to overcome'the variationsin the output level of the head.

Curve-B (Figure 7) shows the response characteristics obtained with a. typical electromagnetic transducer head assembly of the prior art, namely, an assembly having a core portion defininga single non-magnetic gap across which the medium rides and a coil surrounding that core portion. As will be evident from Figure '7, the response of the head rises and falls in the region from 20*to almost 1000 cycles. This characteristic results from the effect of magnetized portions of the wire approaching the edge of the head and alternately bucking or boosting the flux associated with movement of the medium across the non-magnetic gap. It is extremely difficult if not impossible to compensate for these rises and falls by equalizing networks in the magnetic reproducingequipment. For this reason magnetic recorders using such heads have not provided uniform frequency response. Figure 8 isa diagram showing the results of a further test indicative of the performance of the head of the subject invention. In this test, a 20 cycle per second saturating square wave was recorded onra wire traveling at 4 feet per second. Signals were picked up with various heads, ampli, fied, andapplied to the vertical deflection plates of anoscilloscope. A saw-toothsweep was applied tov the horizontal plates, thusproducing an apparently stationary picture. of. the, induced voltageinrthe winding of the head.

Thecurve, A (Figure 8) shows the nature of the voltageinduced in the winding of a closedtype electromagnetic transducer head having a coil encircling the non-magnetic gap itself when the medium described above travels over the nonmagnetic gap thereof. In ahead of this type the flux associated with approach of the medium to the; head tends to encircle the portions of the core other than those portions encircled by the coil. and hence tends to produceno voltage. As willbe; evident from curve A, the reproducedsignal corresponds almostexactly to the time rate of; variatinnof. the recorded si nal, theonly dc..-

parturefrom; mist-characteristic. being the small pipsiollowing. each major pip andwhich are, due to, crossing of the. magnetic. reversals. from erase.

head laminations to the record-pickup head laminations. The latter effects can be eliminated if desired.

It is, of course, well recognized that a closed type head such as that for which curve A. Wi ure 8) was obtained has very substantial disadvantages in practical magnetic recorders because of the difiiculty of threading the medium through the head.

Curve B (Figure 8) is a curve taken under the same conditions as curve A but with the head for which the curve B (Figure '7) was obtained. It will be evident from this figure that the reproduced Voltage departs substantially from the time rate of variation of the recorded signal and contains many spurious dips and bumps. These correspond to the uneven response of this head to various frequencies since if a uniform response were achieved the curve B would correspond identically with the time rate of variation of the recorded signal.

Curve C (Figure 8) shows the induced voltage in the head of the present invention under the same conditions as curves A and B. As will be evident from this curve, the departures from the perfect response curve are very greatly reduced and the resultant voltage corresponds very nearly with the perfect response. This is due to the fact that the initial and final contacts of magnetic reversals of the wire with the head create flux changes threading both coils 2 and to produce no resultant voltage. When a reversal is moving in the region between an initial. and final contact with the head, the proportion of the flux through each coil changes, thus producing the slow rises and falls of voltage between the desired pips.

In Figure 9, an alternative embodiment of the present invention is shown. As indicated in this figure, the core 253 comprises a Ll -shaped portion and an O-shaped portion, each defining a common air gap 22. The single coil 2 encircles this air gap and is directly connected to the reproducer 21. This head functions to eliminate most of the effects of stray magnetic fields and the effects due to the lengthy magnetizable medium approaching the air gap 22 since it defines a shunt magnetic path about the coil 2% which has a varying proportion of the total flux passing through it as tne position of the magnetic variation in the medium changes. A non-magnetic gap 53 is provided in the lower portion 2%. The structure shown in Figure 9 has been found advantageous under certain circumstances, but is not considered to give the outstanding advantages of the structure shown in Figure 2.

In the appended claims I have described the direction of windings 2d and 2? as in such direction as to produce opposing voltages upon change in the flux of the air gap 22. By reference to Figure 3, it will be evident that when the flux traversing the coils 24 and 25 is in the direction of the arrows 38 and M3 (the direction to which the output voltage is in opposition), the flux across the air gap corresponding to the arrows 38 and Ml is oi like direction.

In the foregoing specification I have described the structure of the present invention in detail and in addition I have explained what I believe to be its mode of operation. It will, of course, be understood that I do not wish to be limited thereto since many modifications both in the circuit arrangement and the structures disclosed may be made withcut'departing from the spirit and scope of my invention. I, of course, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

I claim as my invention:

1. An electromagnetic transducer head assembly for use with a lengthy magnetizable record medium including a core member defining two magnetic loops having spaced portions as well as a common non-magnetic gap, said loops being integrally joined at portions near said common gap, and a pair of windings each encircling a, respective one of said loops said windings being electrically connected in series relationship to produce opposed induced voltages therein upon change in the fluX of said gap.

2. An electromagnetic transducer head assembly for use with a lengthy magnetizable record medium including a core member defining two magnetic circuits having spaced portions and a common non-magnetic gap, said circuits being integrally joined at portions near said common gap, one of said circuits having relatively great reluctance when viewed from said gap as compared to the reluctance of the other, a pair of windings each linking a respective one of said circuits, and electrical circuit means connecting said windings in series relationship to produce opposing voltages upon change in flux across said gap.

3. An electromagnetic transducer head assembly for use with a lengthy magnetizable record medium including a magnetic core member defining two magnetic circuits having spaced portions and a common non-magnetic gap, said circuits being integrally joined near said common gap and on each side thereof, one of said circuits having a relatively great reluctance when viewed from said gap as compared to the reluctance oi. the other, said one circuit including opposed magnetic portions adjacent said gap to guide said medium through said non-magnetic gap, and a winding linking said other circuit.

l. An electromagnetic transducer head assembly for use with a lengthy magnetizable record medium including a core member defining'two spaced magnetic circuits having a common nonmagnetic gap, said circuits being integrally joined near said common gap, one of said circuits having relatively great reluctance when viewed from said gap as compared to the reluctance of the other, said one circuit including opposed portions adjacent said gap to receive said medium, and a pair of windings each linking a respective one of said circuits, said windings being electrically connected in series and oppositely wound to produce opposing induced voltages therein upon change in the flux of said gap.

5. An electromagnetic transducer head assembly having a core member defining a non-mag netic gap and two individual magnetic circuits therethrough, one of said circuits comprising a relatively short length of magnetizable material shunting said gap and the other a relatively long length of magnetizable material defining portions adjacent said gap to receive said medium as it approaches and leaves said gap and portions of said core member connecting said circuit portions, a winding linking said one magnetic circuit, a winding linking said other magnetic circuit, said windings being connected in series and oppositely wound to produce oposed induced voltages upon change of flux through said gap, said windings being proportioned to produce negligible net voltage when a magnetic change in said medium approaches or leaves said head assembly.

'6. An electromagnetic transducer head assembly having a core member defining a non-magnetic gap and two individual magnetic circuits of substantially equal cross-section, said circuits being joined near said gap, one of said circuits comprising a relatively short length of magnetizable material shunting said gap and the other a relatively long length of magnetizable material of greater reluctance than said short length circuit defining opposed portions adjacent said gap to receive said medium as it approaches and leaves said gap and a magnetic circuit connecting said portions, and a winding linking said one circuit.

'7. An electromagnetic transducer head assembly for use with a lengthy magnetizable record medium, including a first magnetic core portion defining a non-magnetic gap and having portions adjacent said gap to receive said medium as it approaches and leaves said gap, a second magnetic core portion having greater magnetic reluctance than said first magnetic core portion and shunting said gap, and a winding encircling said second magnetic core portion.

' RAYMOND E. ZENNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 654,390 Fessenden July 24, 1900 1,943,899 Muller Jan. 16, 1934 2,130,871 Boehne Sept. 20, 1938 15 2,325,844 Fischer Aug. 3, 1943 2,418,542 Camras Apr. 8, 1947 FOREIGN PATENTS Number Country Date 429,987 Great Britain June 11, 1935 

